An X-ray CT apparatus calculates an X-ray absorption coefficient (CT value) of each point in a subject from measured projection data obtained by imaging the subject from multiple directions to obtain a distribution image (CT image). As a technique for achieving both of low radiation exposure of a subject to X-ray and high image quality of the subject, there is a technique called an iterative reconstruction method (see, e.g., Patent Document 1). In the iterative reconstruction method, in order to make measured projection data obtained by an X-ray CT apparatus and calculated projection data calculated from a CT image generated from the measurement data equal to each other, the calculated projection data or the CT image is iteratively corrected, thereby reducing noises of the CT image at a low dose.
The iterative reconstruction method requires bigger calculation amount with iterative updating than an analytical method of calculating a CT value in the related art and also requires optimization of huge parameters. In particular, the iterative reconstruction method requires setting of many parameters to perform a smoothing process between adjacent pixels during updating to obtain a noise reduction effect.
As a method of reducing the calculation amount, there is a method of stopping iterative correction on a CT image output during the iterative correction or changing parameters at the point of time when a noise of the CT image, here, a measurement value of the standard deviation (hereinafter referred to as “SD”) representing the variation of a CT value, reaches a desired value (see, e.g., Patent Document 2).